The fifth-generation fighter is intended to replace America’s aging warplanes with three variants: a conventional take-off and landing jet, an aircraft carrier version and a short take-off and vertical-landing model.

The Government Accountability Office estimates the project is running 42 percent over budget. The GAO figures the F-35 program will cost $395.7 billion, “an increase of $117.2 billion from the prior 2007 baseline.”

But the Project on Government Oversight, a defense watchdog group, says GAO badly underestimated the costs by using the wrong baseline year for its calculations.

“The cost documentation of the F-35 program started in 2001, not 2007,” says Winslow Wheeler, director of the Straus Military Reform Project and Center for Defense Information at POGO.

“Set in 2001, the total acquisition cost of the F-35 was to be $233 billion,” he points out. That means the program’s price tag has jumped $162.7 billion — or 70 percent.

But there’s more, Wheeler says.

“The original $233 billion was supposed to buy 2,866 aircraft, not the 2,457 currently planned,” he says.

Adjusting for the shrinkage in the fleet, Wheeler calculates the unit cost growth at 93 percent — nearly double the Defense Department’s original estimate.

Even by the GAO’s conservative estimates, the long-term operational costs of the fighter jet will top $1 trillion.

All in all, the Defense Department pegs the F-35’s operating cost at $1.1 trillion for all three variants based on a 30-year service life.

“That’s unaffordable and simply unacceptable in the current fiscal environment,” the GAO states.

A spokesman for the Bethesda, Md.-based Lockheed Martin declined to comment on deadline. Northrop’s spokesman did not respond to Watchdog.org Virginia Bureau’s request for comment.

Skeptics continue to question the F-35’s bang for the buck.

Analyzing the conventional take-off and landing model, the GAO calculated the jet’s operating cost at $35,200 per flight hour, compared to $22,500 for the F-16.

Lockheed, in a recent news release, said the stealthy, radar-evading F-35 — with versions designed for the Air Force, Navy and Marine Corps — is performing well in initial tests.

As of June 30, Lockheed reported the F-35 had conducted 595 test flights in 2012 versus a plan of 445, and had accrued 4,830 test points against a plan of 3,901.

Wheeler says a more accurate comparison is with the F-22, which debuted in 2005. He says the F-35’s operational costs could be comparable with that jet.

The United States, United Kingdom and seven international partners — Australia, Canada, Denmark, Italy, the Netherlands, Norway and Turkey — are contributing to the development costs of the single-engine F-35.

Purchases by the U.S. armed forces and these partner countries combined with foreign military sales are expected to exceed 4,000 total aircraft, according to the Virginia-based Northrop Grumman.

On its website, Northrop says it has awarded some of its most significant subcontracts to firms overseas, including an agreement to produce at least 400 center fuselages in Turkey.

Though critical of the costs, Wheeler says there is a demonstrable need for a new American jet fighter.

“The U.S. air combat inventory desperately needs new aircraft. The current inventory is aging and shrinking fast at increasing cost,” he told Watchdog.org Virginia Bureau.

“We should not presume to think that all future enemies will either have no air force — such as Afghanistan and al Qaeda — or an incompetent one, like Iraq.”

The National Defense Industrial Association says that Russia and China are both developing fifth-generation stealth fighter jets — the Sukhoi T-50 and Chengdu J-20 — to rival America’s Air Force fighter fleet, which NDIA says “is smaller and older than at any time since the service was created in 1947.”

But, Wheeler said, the ballooning cost of the F-35 are “making the problems worse.”

“Given the failure of the $200 billion-plus the Air Force received from 2001 to 2009 to stem the shrinking, aging service, there is no reason to think that throwing still more money at the Air Force will do anything but perpetuate the problems,” Wheeler said.

Kenric Ward is a veteran journalist who has worked on three Pulitzer Prize-winning newspapers. A California native, he received a BA from UCLA (Political Science/Phi Beta Kappa) and holds an MBA. He reported and edited at the San Jose Mercury News and the Las Vegas Sun before joining Watchdog.org in 2012 as Virginia Bureau Chief.